Tunable lasers are extensively used as fundamental tools in the field of laser spectroscopy. However, problems associated with frequency variations of the laser light arise which tend to degrade the operation of the laser system. For example, dye-lasers, which employ organic media, suffer from variations in the media that result in undesirable frequency shifts and noise. To solve these problems, various means have been proposed for stabilizing the laser frequency.
Although the following description will be directed to the dye-laser for purpose of explanation and example, it should be understood that the invention is not limited thereto.
In a system employing a dye-laser, one technique for achieving frequency stabilization is to tune the dye-laser frequency relative to a fixed reference cavity by using piezo transducers to change the cavity lens mechanically. Nevertheless, the response of such a system is not sufficient to compensate for rapid phase fluctuations, such as one rad per microsecond, which is generated by a typical dye jet.
External laser frequency stabilization has been demonstrated and implemented in prior art apparatus. In one approach, frequency fluctuations are corrected by means of an external transducer after the radiation or light has left the laser. The corrected frequency is monitored by a passive reference cavity, and an amplified and integrated error signal is fed back to a frequency transducer. In such prior art apparatus, an acoustooptic modulator Doppler shifts the frequency of the laser beam by reflecting it off a traveling acoustic compression wave. The shift in the first diffraction order simply equals the frequency of the acoustic wave. However, acousto-optic modulators have an unavoidable time delay, of the order of a microsecond, which limits the possible bandwidth of the servo loop, and thus also limits the low frequency gain that is realized without stability problems. Whereas such a delayed response has been found sufficient for frequency stabilization of an argon-ion laser, it is not fast enough to compensate effectively for rapid phase fluctuations that are encountered in a typical cw dye laser.
In a dye-laser, thickness variations of the dye jet as small as a few molecular monolayers introduce phase shifts of several radians during the time the dye travels through the beam waist, which may be approximately 3 microseconds. Variations in beam diameter change the light path in the reference cavity leading to detection error. Frequency stabilizers capable of compensating for such rapid fluctuations generally rely on the rapid response of an electro-optic phase modulator inside the laser resonator. However, installation of a modulator inside the cavity of a commercial dye-laser requires difficult optical and mechanical modifications, and tends to lead to a substantial reduction in power, as a lossy transducer is inserted at a location of high light intensity.